The present invention relates to a drilling arrangement in general, and more particularly to a drilling arrangement which is capable of forming undercut holes in support structures.
It is already known to attach objects, especially heavy objects to support structures, such as walls, ceilings or floors, by using anchoring bolts accommodated in pre-drilled holes in the support structures. The anchoring bolt may be secured in its hole either by spreading or expanding expandable portions of the anchoring bolt, or by cementing the anchoring bolt in its associated hole. Even though it is already known that the resistance which the anchoring bolts of both above-mentioned types offer to forces which attempt to extract such anchoring bolts from their associated holes can be substantially increased by giving the holes an undercut configuration, the customary practice nowadays is to accommodate such anchoring bolts in cylindrical holes which correspond in shape to the anchoring bolts. The most likely reason for this situation is that the forming of undercut holes in the support structures brings about substantial problems and, up to now, necessitated the utilization of complex drilling tools.
An exception to the above-mentioned situation is disclosed in the German published patent application DOS 2,349,998. This publication illustrates and describes a drilling unit or arrangement which has a tool element equipped with a collar. When it is desired to undercut a pre-drilled cylindrical bore in the support structure, the collar is pressed against the support structure at the region of the open end of the pre-drilled cylindrical bore so as to form a fulcrum for pivoting the tool element. The pivoting may be accomplished either in a rocking fashion, or in a conical orbiting fashion, and in either event the drill bit which is mounted at the leading end of the tool element will penetrate into the material of the support structure which surrounds the originally cylindrical bore, thus removing such particulate severed material from the support structure and from the bore through the open end thereof, and forming an undercut bore.
Excellent reasons have been achieved by the use of this drilling arrangement for forming undercut holes in relatively rigid or high-strength materials, such as concrete or the like. However, when it was attempted to use this drilling arrangement for forming undercut holes in support structures of relatively weak or low-strength materials, the results have been disappointing. This is attributable to the fact that such low-strength materials are not rigid enough to be capable of withstanding the forces which act on the material of the support structure during the rocking or orbiting motion of the tool element, particularly since the fulcrum and thus the region of application of such forces is located at the open end of the bore. The result of this is that the open end of the bore is either gradually enlarged, or a break-away cone is formed at the open end of the bore, which has basically two disadvantageous consequences. First of all, the support of the collar at the enlarged open end of the bore is less than reliable. Secondly, and even more importantly, the enlargement of the open end of the bore, and particularly the formation of the break-away cone substantially reduces the resistance of the support structure to the extraction of the anchoring bolt which is supported in the bore, from such bore.